1. Field of the Invention
The present invention relates to an anode material of a lithium-ion secondary battery and a preparation method thereof, and more particularly to an anode material of a lithium-ion secondary battery, which is a powder material made by mixing a natural graphite, an artificial graphite or both, and then mixing one or more types of resin solutions of a high hard carbon content to the mixture, and processing the mixture by a mist spray drying process and a carbonization heat treatment, and covering the mixture by a special resin material, as well as a process method of the anode material of the lithium-ion secondary battery.
2. Description of Related Art
In recent years, anode materials of lithium-ion secondary batteries are studied extensively, since there are many existing problems of the lithium metal used as an anode material for lithium batteries, and one of the problems is the dendritic crystal precipitated on the surface of the lithium metal, not only causing a safety concern, but also affecting the cycle life of a battery, or even causing a failure of the battery. Carbon system is one of the most popular applications, and graphite is generally used as an anode material for manufacturing a commercial lithium-ion secondary battery, and graphite can be mainly divided into artificial graphite and natural graphite. For artificial graphite, mesocarbon microbead (MCMB) carbon comes with a complicated manufacturing process and adopts a carbonization furnace for its processing, and thus the artificial graphite incurs a high production cost. For natural graphite, a greater irreversible capacity at the first cycle generally occurs in the process of charging or discharging a battery, and a surface modification is generally used at present to overcome such shortcoming, wherein a carbon containing layer is coated onto the surface of the graphite, and a carbonization heat treatment is conducted to form an amorphous carbon material, and this layer of amorphous carbon material can suppress lithium complexes from being inserted between the graphite layers to reduce the irreversible capacity. Although the way of coating a pitch onto the surface of graphite provides a smaller specific surface area, a less first irreversibility, a higher quality of the anode material of graphite, a better compatibility with electrolyte solutions, and a lower cost, yet the capacity is reduced with the number of times of charges and discharges, and thus causing a shorter cycle life. For example, an oxidation treatment of graphite as disclosed in Japan Patent Publication No. 2000-261046 changes the status of the surface of graphite to improve the compatibility of an anode material with an electrolyte solution, yet its capacity is lower than the capacity of pure natural graphite. As disclosed in P.R.C. Patent Publication Nos. CN1224251A and CN1304187A, ethanol or an equivalent solvent is used for dissolving a resin of a high hard carbon content such as furan resin, polyacrylonitrile resin, phenolic resin, urea resin, epoxy resin, polyester resin, polyamide resin and melamine resin to coat the graphite, but such method has the drawbacks of a too-large specific surface area, so that the coated graphite particles may be stuck together to form lumps, and the graphite lumps will cause a peel-off or damage of the coated layer after the grinding process takes place and affect the performance of the anode material.
Based on the study of the aforementioned methods of adopting a resin of a high hard carbon content to change the status of the surface of natural graphite, the first irreversibility can be reduced, and a better capacity retention can be achieved in a charge/discharge cycle, and the amorphous carbon coating can suppress lithium complexes from being inserted between into the graphite layers to reduce the irreversible capacity and retard the charge/discharge latency. From the study, it is found that natural graphite comes with a sheet structure, such that the insertion and extraction of the lithium ions are restricted by the crystal boundary of the graphite, and a lower quick charge/discharge performance is resulted. Furthermore, there are still many crevices on the surface coated with amorphous carbon, and thus resulting in a too-large specific surface area of the graphite.